For many teenagers, getting a phone is more important than getting a driver's licence. Photo: New York Times

The change is occurring across young age categories, with sharp drops among teenagers. The share of 17-year-olds with driver's licences, for example, was down to 45 per cent in 2014, from 55 per cent in 2001. And the rates are falling for young men and young women alike.

Total vehicle distances driven by young drivers have plummeted, not only because a smaller share of them are driving but also because distances per young driver have also declined. In 1990, each driver aged 16 to 19 drove an average of almost 13,600 kilometres per year. In 2009, it was less than 10,400. Thanks to this and to safer cars, fatal car accidents involving teenagers are falling sharply, even though they remain the leading source of death in that age category. And fewer drivers also generally means less greenhouse gas emissions.

Why is this happening? One factor is the rise of laws that limit teens' ability to drive by themselves, which might make getting a licence less appealing. A related possibility is that the changes in the driving laws are causing faulty measurement. Yet these things can't explain the decline among 20 to 24-year-olds.

So we are left with three explanations. One, favoured by researchers at the University of Michigan, is the rise of social media. If you can connect with friends over Snapchat and Instagram, there's less of a need to get together by driving to popular hangouts or by cruising. The second is economic. Teenagers asked why they don't have a licence often cite the expense.A final possibility is ongoing urbanisation; as Americans increasingly live in cities, it's easier to get by without a licence.

Whatever the causes, the change is stark. For many young people today, there's a new rite of passage that's bigger than learning to drive: getting your first phone.